1. Field of the Invention
The present invention relates to a liquid crystal display device that displays an image by inverting the polarity of image data applied to the picture element electrodes of a liquid crystal display panel at certain intervals of time, and a circuit and a method for driving the liquid crystal display device. More specifically, it relates to an active matrix liquid crystal display device having a switching device for each picture element, and a circuit and a method for driving the liquid crystal display device.
2. Description of the Prior Art
An active matrix liquid crystal display panel comprises two glass substrates with a liquid crystal sealed therebetween. On one of the glass substrates, a plurality of picture element electrodes arranged in horizontal and vertical directions, and a plurality of switching devices for turning on and off the voltage applied to each picture element electrode are formed. As the switching device, a thin film transistor (to be referred to as xe2x80x9cTFTxe2x80x9d hereinafter) is often used.
Meanwhile, on the other glass substrate, color filters and a counter electrode are formed. These two glass substrates are disposed in such a manner that the surface on which the picture element electrodes are formed faces the surface on which the counter electrode is formed with each other. The color filters are classified by three colors, i.e., red (R), green (G) and blue (B), and the R, G and B color filters are arranged in a predetermined order such that one color filter corresponds to one picture element electrode. In the following description, a substrate having the picture element electrodes and TFTs will be called a xe2x80x9cTFT substratexe2x80x9d, and a substrate having the color filters and the counter electrode will be called a xe2x80x9ccounter substratexe2x80x9d.
Further, a pair of polarizing plates are disposed such that the TFT substrate and the counter substrate with a liquid crystal sealed therebetween are sandwiched between the polarizing plates. The pair of polarizing plates are generally disposed such that polarizing axes cross each other at right angles.
The active matrix liquid crystal display panel is driven by an alternating voltage. That is, with the voltage applied to the counter electrode being a reference voltage (common voltage), a voltage which switches between positive polarity (+) and negative polarity (xe2x88x92) at certain intervals of time is supplied to the picture element electrode. The voltage applied to the liquid crystal preferably has a positive voltage waveform and a negative voltage waveform, which are symmetric. However, even if an alternating voltage having a positive voltage waveform and a negative voltage waveform which are symmetric is applied to the picture element electrode, the positive voltage waveform and negative voltage waveform of the voltage that is actually applied to the liquid crystal are not symmetric. Therefore, the light transmittance when a positive voltage is applied differs from the light transmittance when a negative voltage is applied, whereby luminance fluctuates at the period of the alternating voltage applied to the picture element electrode, resulting that the phenomenon called xe2x80x9cflickerxe2x80x9d occurs.
As conventionally used methods for controlling the occurrence of a flicker, there are known methods such as a method in which the voltage applied to the counter electrode is changed, a method in which the polarities of voltages applied to picture element electrodes adjacent in a horizontal or vertical direction are made different, and a method in which the frequency of inversion of polarities is made high. These techniques are disclosed in Japanese Patent Laid-Open Nos. 113129/1987, 34818/1990, 149174/1994, 175448/1995 and 204159/1997, for example.
When voltages of different polarities are applied to adjacent picture element electrodes, there can be used (1) a method in which voltages of one polarity are applied to picture element electrodes arranged in a vertical direction and voltages of the other polarity are applied to picture element electrodes adjacent in a horizontal direction, (2) a method in which voltages of one polarity are applied to picture element electrodes arranged in a horizontal direction and voltages of the other polarity are applied to picture element electrodes adjacent in a vertical direction, and (3) a method in which voltages of opposite polarities are applied to picture element electrodes adjacent in vertical and horizontal directions. A pattern which shows the polarities of voltages applied to the picture element electrodes of a liquid crystal display panel is called a xe2x80x9cpolarity patternxe2x80x9d.
However, the flicker becomes conspicuous, when a vertical-stripe pattern is displayed with the polarity pattern of the above (1), when a horizontal-stripe pattern is displayed with the polarity pattern of the above (2), and when a mosaic pattern (checker pattern) is displayed with the polarity pattern of the above (3).
In Japanese Patent Laid-Open Nos. 297831/1993, 69264/1996 and 95725/1999, it is proposed that one polarity pattern is switched to another according to image data supplied to adjacent pixels. In the methods disclosed in these gazettes, a plurality of different polarity patterns are made available, and one polarity pattern is switched to another when the image data supplied to two adjacent picture elements have a particular relationship.
However, in the case of the above-described conventionally used methods for switching one polarity pattern to another, the polarity patterns are switched from one to another even when a predetermined pattern is present in a very small portion of a display screen. Therefore, the switching of polarity patterns frequently occurs, resulting in only a reduction in display quality.
The object of the present invention is to provide a liquid crystal display device that can reduce or prevent the occurrence of a flicker more surely and does not switches polarity patterns unnecessarily so as not to cause a reduction in display quality, and a method and a circuit for driving the liquid crystal display device.
The liquid crystal display device of the present invention, as exemplified in FIG. 8, comprises a liquid crystal display panel (13) having a plurality of picture elements arranged in horizontal and vertical directions, an image data output section (11) that outputs image data (RGB), a flicker-judging section (12) that detects the difference in gradation between the image data (RGB) supplied to picture elements of the same color of two pixels adjacent in a horizontal direction and judges whether a flicker occurs or not based on the result of the detection to output a polarity pattern switching signal (FLK), and a polarity image data-supplying section (14) which supplies the liquid crystal display panel (13) with the image data (RGB) outputted from the controller (11) with the polarities based on the polarity pattern corresponding to the polarity pattern switching signal (FLK).
The liquid crystal display device of the present invention has a flicker-judging section in which the difference in gradation between the image data of two pixels adjacent in a horizontal direction is detected by each picture element of the same color. When the difference in gradation between the image data of picture elements of the same color of two pixels adjacent in the horizontal direction is large, the size relationship between the image data of the two pixels is examined, and when the same size relationship repeats in between the pixels in the horizontal direction, it is concluded that there is a fear of occurrence of a flicker.
Thus, in the liquid crystal display device of the present invention, the polarity pattern is changed according to the image data. Therefore, the occurrence of the flicker can be prevented surely.